According to the preamble of claim 1, the invention relates to a shifting device for variable-speed transmissions of motor vehicles.
In such shifting devices, the rotary motion of the gear shift rods serves to pre-select the individual gear shift gates and, in general, requires no great shifting forces. The axial longitudinal motion of the gear shift rod serves to introduce the desired gear and requires stronger shifting forces, particularly in transmission for heavy vehicles and such vehicles where the driver""s seat is situated far from the transmission.
Industrial vehicles, like existing buses and trucks, have spatial conditions necessarily for installing the transmission far from the driver""s sear with the design of their forward control, under which the transmission jerks. The distance is especially great in vehicles having under-floor or rear engines. The precise change of gear is often made difficult by long, sometimes rigidly extending, gear shift linkages in a mechanically shifted transmission.
In order that the driver of a motor vehicle can fully turn his attention to the street traffic, he must be, as much as possible, unburdened and assisted in all the actions needed to guide a motor vehicle
Every truck driver knows how decisive the perfect control of the variable-speed transmission can be in difficult traffic situations. Pneumatic gear shift aids for industrial vehicles of all sizes can provide relief.
Formerly known servo-gear shift systems are partly attached directly to the transmission and have a control or piston rod accessible from the outside. The gearshift linkage is connected with the control rod. The assistance is activated via a longitudinal motion of the control rod. This type of control is found associated with 2-rod or cable-pull gear shifts. A disadvantage here is the sealing of the control and piston rod by rubber boots and the lack of lubrication. In trucks, this place is more exposed to dirt. When changing the ratio of the gear shift rod, the beginning of the servo-assistance also changes or it must be adapted to the linkage ratio by modifying the valve. The same also applies to gear shift aids consisting of one control valve and servo-cylinder separated from each other. The valve and cylinder are connected by ball joints with the gear shift lever and a bracket which, in turn, is fastened on the transmission. This arrangement still has the added disadvantage that upon each gear shift the parts move relative to the transmission and chassis of the vehicle and thus the air pipes, which the valve and cylinder are interconnected, can wear by rubbing.
Such pneumatic gear shift aids in broken-up design are already known, which consists of one mechanical pneumatic control part and one separate pneumatic power part. A servo-control in broken up design is known from Loomann; Zahnradgetriebe; 2nd edition; Springer Editing House; 1988, p. 225. The control part is a mechanically actuated control valve actuated by a gear shift linkage. When changing gear, the selector motion is mechanically transferred directly to the transmission. During transmission of the gear shift movement, the control valve is actuated and, at the same time, the manual shifting force is mechanically transferred via a lever to the transmission. In addition, the manual shifting force is pneumatically assisted by a compressed-air cylinder. At the same time, the compressed air cylinder forms the pneumatic power part as a two-position cylinder with integrated hydraulic damper. A direct proportional representation of the manual shifting force is not obtained here. The paths between control part and power part are long and the installation takes up much space. Damage to the compressed air lines, between control part and power part, cannot be prevented.
EP 0 251 807 has disclosed a transmission where the manual shifting force of the driver is assisted while the selector force is transferred to the transmission without assistance. While the shifting movement is transmitted to a first gear shift shaft, the selector movement is reversed and axially moves a third shaft which is independently rotatable of the first gear shift shaft that transmits the manual force. The servo-assistance is triggered by the first gear shift shaft to the servo element and transmitted by a second gear shift shaft, which is rotatable independently of the first gear shift shaft, to the third gear shift shaft and from there to the gear shift rails. The second and third gear shift shafts are non-rotatably connected with each other, the third gear shift shaft being axially displaceable relative to the second gear shift shaft.
DE 195 39 471 has disclosed a variable speed device where the control valve and the power part are combined to form one structural part. The shifting force is increased in proportion to the driver""s shifting force and achieves the same effects as in a hand shifted transmission. The driver does not loose the feel of the gear shift, he can immediately detect a sense of whether a gear has been introduced or how long the synchronization phase lasts. In the gear shifting device, the transmission of the forces to the individual parts remains in need of improvement.
The problem on which the invention is based is to provide a gear shifting device which makes a simplified and reliable shifting with servo-assistance possible while improving the transmission.
The problem is solved by a shifting device having the features of claim 1. Development are object of sub-claims.
According to the invention, in a shifting device firmly mounted on the transmission housing, a central gear shift shaft is provided which is connected to the gear shift linkage actuated by the driver and rotatably actuatable by it. With the gear shift shaft, the manual shifting force is transmitted to the disengaging lever of the assistance device and to the gear shift rails of the transmission. The gear shift shaft can be displaceably actuated by the gear shift linkage to select the gear shift rail to be shifted. The shifting force, supplied by the assistance device, can be transmitted to a hollow shaft which is firmly situated axially in the shifting device, surrounds the central gear shift shaft and is rotatable with the central gear shift shaft to shift the gear shift rails.
In an advantageous development, axially aligned grooves in which the pins engage are provided in the central gear shift shaft. Thereby is formed a non-rotatable, but axially movable connection of the central gear shift shaft with the disengaging lever of the assistance device.
An advantageous development shows the pins rotatably supported in the disengaging lever of the assistance device, the rotatable support of the pins being advantageously provided by needle bushings.
Still one other advantageous development provides a rotatable means between the disengaging lever of the assistance device and the control rod of the assistance device, the rotatable means advantageously being a roller situated on the disengaging lever.
In another advantageous development, the gear shift shaft has in the area of a connection of central gear shift shaft and surrounding hollow shaft openings to accommodate connecting elements between the central gear shift shaft and the hollow shaft. The openings are designed larger than the connecting elements in order to make possible a slight torsion of the gear shift shaft before the hollow shaft is moved along.
The shifting device can be designed as one structural part which, as a complete unit, is adaptable to the transmission housing.
The invention solves the problem of sealing of the longitudinal movement and modification of the valve, the same as the parts moving relative to their surroundings. The mechanics to control the servo-unit and emit the servo-force is supported in a housing in such a manner that by a rotary motion on the gear shift shaft the servo-assistance is activated and is transmitted, via another lever, to a hollow shaft and from there to the transmission. The gear shift shaft is sealed by means of shaft packing rings with scraper and protective cap. In addition, the parts are lubricated via an oil spraying pipe. The gear shift rod ratio, which differs according to type of vehicle and customers, can be adapted to the valve of the servo-unit via a gear shift lever fastened on the gear shift shaft by changing the suspension radius. Thus, in a simple manner, it can be ensured that the same valve and also the same servo unit can be used for all applications.
To connect the pneumatic shifting device to the gear shift linkage rigidly possible, the latter must make possible the following operations:
transfer to the valve of the manual shifting force;
transfer to the transmission of the servo force+manual shifting force dependent on the valve control;
mechanical bridging of the servo-assistance unit;
transfer of the selector forces
adaptation of the shifting device to the shift rod.
The problems here consist in that:
the transfer of the manual shifting force as valve control force must be as free of losses as possible;
the selector force must not increase;
the quick change or replacement of the shifting device must be made possible;
the installation space around the transmission, especially toward the chassis of the vehicle, is extensively limited;
an easy and perfect assembly must be made possible;
the servo-assistance must always be the same independent of the remote control ratio;
the oil supply of the pneumatic assistance device and of the mechanical parts must be ensured.
These problems are taken into consideration in the inventive shifting device.
The shifting movement is aided by compressed air as servo-assistance while the selector movement by hand is carried out without assistance. Due to the fact that only one part of the gear change is aided by a servo-assistance, the construction of the shifting device is simplified compared to completely servo-assisted shifting devices. Costs and installation expenditure are considerably reduced, replacement in transmissions already installed is also possible without problems. In addition, the driver can manually shift when, due to damages in the compressed-air system, the compressed air is too scarce or fails completely. To that end, no shifting steps adapted in any manner are required. The braking capacity of the motor vehicle, insofar as offered by the transmission, remains assured.
The compressed air assistance controls a great part of the force to be spent in shiftingxe2x80x94but only to the extent that the driver retains the shifting feel. The manual force to be applied by the driver is reduced by the pneumatic shifting assistance to a fragment of the totally required force. Synchronized transmissions of industrial motor vehicles change in heavy vehicles also to light mechanical transmissions. When driving on inclined roads, the downshift of the transmission becomes extraordinarily simplified, especially in the lower gears, and thereby a considerable contribution is made to the increase in accident protection.
The inventive shifting device can be used in all synchronized transmissions having H-gearshift, double-H-gear shift and superimposed H-gear shift. It also has a sufficient place for attachment to the transmission in the installation space extremely limited by the manufacturer of the vehicle. In case of a required exchange or replacement, the down times for the vehicle operators are gratifyingly short inasmuch as the whole shifting device can be jointly exchanged or assembled without special adjustments being needed for placement in the vehicle. Likewise during the gear shift process, the driver has a steadily detectable feedback to the shifting cycles in the transmission. Due to the existing mechanical connection between the gear shift lever and the synchronization system in the transmission, the driver detects the reactions of the transmission, the same as in a mechanical and non-assisted shifting device. Thus the driver always retains the possibility of engagement in the gear shift cycle and can always discontinue the development of the gear shifting process. In case of failure of the compressed-air assistance, the driver retains, without limitation, the shifting characteristics of the transmission so that in emergency cases he can brake the vehicle by changes of the ratio certainly at increased expenditure of force but reliably.
By an adequate design of the shifting device, the assistance forces it produces to assist the gear shift can be adapted to the shifting system between driver and transmission. The shifting device is firmly attached to the transmission housing so that no relative movements between transmission and shifting device result. There are no longer any moved lines, valves or cylinders between transmission and shifting device. Universal joints and brackets are no longer needed. The shifting device can be replaced without shortening the gear shift linkage. The installation position of the pneumatic servo-assistance device on the shifting device is optional, it can be plugged in or screwed on.